Apertures for coherent ultrasound two-dimensional (2D) arrays are increasing in size to eliminate the operator dependence of the image acquisition and to provide the high resolution needed for imaging at a distance. However, as aperture size increases, the number of elements increases along with the data collected.
C-mode systems provide intuitive, first-person images and reduce the data collected. Most of the existing techniques for C-mode ultrasound devices have a number of disadvantages when trying to scale to larger scale, lower cost devices. Larger scale devices benefit from larger aperture size, which directly improves resolution. Current Applications Specific Integrated Circuits (ASIC) technology, however, has moved in the opposite direction, to smaller dice, which lead to lower resolutions. It is, therefore, advantageous to utilize thin film transistor (TFT) technology which is scalable to large areas. U.S. Patent Application No. 2007/0016022 and U.S. Pat. No. 7,402,136 describe a technique to coherently image by directly sampling quadrature and in-phase components from a radio frequency (RF) signal. This technique requires a high sampling rate that is not suitable for polysilicon thin film transistor (TFT) at medical ultrasound frequencies. In addition, the transducers are piezoelectric transducer (PZT), mechanically diced and bonded to a flexible circuit, which is difficult to scale to large areas because of interconnect and signal distribution problems. U.S. Pat. No. 5,456,256 describes a 2D solid state array with focusing in transmitter and receiver. Focusing is performed using a time-delay and sum algorithm. The time delay is executed by a high speed multiplexer to select the correct time delay for each element. This technique is difficult to scale to large area arrays because it requires complicated timing between individual pixels for focusing and requires high speed and high precision electronics, especially at medical ultrasound frequencies. U.S. Pat. Nos. 5,483,963 and 6,552,841 describe a 2D focal plane array that uses the same array elements for transmit and receive and also utilizes a lens for incoherent imaging. Complementary Metal Oxide Semiconductor (CMOS) mixers are used for detection of in-phase and quadrature signals. This technique does not teach mixing with low speed electronics or with a diode front-end. Furthermore, there is no teaching on how the common range gate signal may be modified to provide a common reference signal. This technique also requires many high speed CMOS elements that perform a lot of processing operations before the data may be stored. This technique, therefore, is not suitable for polysilicon TFT arrays. None of these techniques are scalable for large areas, low-cost imagers.